NSURLSession delegateQueue Silently Fails if Passed The Main Queue

Of important note: If you want your callbacks to come on the main queue, you
cannot create an NSURLSession withsessionWithConfiguration:delegate:delegateQueue: and pass[NSOperationQueue mainQueue] as the delegateQueue argument. All requests
will silently fail, and never return if you do.

Instead you must continue to us dispatch_async(dispatch_get_main_queue(), ...) or [[NSOperationQueue mainQueue] addOperationWithBlock:...].